The invention relates to a detector cell, especially for measuring the absorption of ultraviolet and/or visible radiation.
Detector cells are used in analytical chemistry in large number for measuring the absorption of a sample in the ultraviolet (UV) and/or visible region of the electromagnetic spectrum. Where applicable, excited fluorescence, chemiluminescence or absorption in the infrared (IR) region of the spectrum are also measured. In addition to being used in sample measuring techniques, environmental analysis and medical diagnostics, detector cells are increasingly being used for absorption measurements especially also in analytical separation methods, such as, for example, liquid chromatography (LC), capillary electrophoresis (CE), supercritical fluid chromatography (SFC) or flow-injection analysis (FIA). Such detector cells are usually in the form of flow cells. These have an inlet opening for the sample to be analysed, and an outlet opening which is often arranged approximately opposite the inlet opening. A measuring chamber arranged between those openings is usually equipped with two measuring windows lying opposite each other. Radiation is passed in via one of the measuring windows onto the sample flowing through the flow cell and, after passing through the sample, is picked up and detected at the opposing measuring window. Measuring cells of that kind are described, for example, in U.S. Pat. No. 3 810 695 or also in U.S. Pat. No. 4 243 883. They are intended for on-line use in production plants and the like and have a measuring chamber of relatively large volume. These flow cells, however, are not suitable for use in analytical separation methods since, owing to the large volume of the measuring chamber, re-mixing and uncontrollable spreading of the separated components of the sample within the carrier medium would occur.
The desire for a reduction in the measuring volume therefore resulted in the development of so-called flow cuvettes. These are essentially a scaled-down version of the known flow cells. Such cuvettes usually have measuring volumes ranging from some hundreds of nanoliters up to a few milliliters. Measuring cells of this kind are described, for example, in a publication by Edward S. Yeung and Robert E. Synovec in Analytical Chemistrry, vol. 58, No. 12, October 1986, pages 1237A-1256A. Suitable construction of the flow region--approximately Z-shaped in the embodiment described--and somewhat slanting irradiation of the sample in the measuring cell region ensure that the measuring radiation travels as long a path as possible in the sample in order to achieve the greatest possible probability of absorption. Despite those measures, detector cells constructed in that manner do not produce the desired results with the desired resolution and accuracy.
An alternative kind of detector cell is the so-called "on-column" detector. In these detector cells, a piece of a capillary tube, or the capillary tube of the mentioned analysis system itself, is used as the boundary for the measuring volume. A measuring cell of this kind is described, for example, in EP-A-0 326 511. It comprises a capillary tube which is fastened in a two-part holding device. The measuring light is introduced perpendicular to the axis of the capillary tube through openings provided for that purpose in the holding device, and the transmitted portion of the measuring light is picked up again and detected at the opposite side of the capillary tube. Usually, the measuring light of a source of measuring light arranged outside the measuring cell is supplied to the capillary tube via fibre-optical light guides and is also conveyed away in that manner. The sources of measuring light used are conventional spectroscopic light sources or also coherent light source lasers. Although the volumes of the measuring region in such "micro"-flow cells are very small, being typically in the range of a few nanolitres, owing to the short interaction path, which, of course, substantially corresponds to the internal diameter of the capillary tube used, restraints are placed on the detection limit (defined as the smallest measurable concentration of the eluted sample molecule). The internal diameters of the capillary tubes are typically about from 10 .mu.m to 50 .mu.m.
In order to remedy this shortcoming in the so-called "on-column" detectors, it is proposed by Xiaobing Xi and Edward S. Yeung in Analytical Chemistry, 1990, 62, pages 1580-1585 or also in EP-A-0 089 157 that the measuring light of a laser source be passed through the capillary tube in the direction of flow. In this manner, the interaction path is lengthened by a factor of up to 1000 as compared with the method in which the light is introduced perpendicular to the axis of the capillary tube.
In this proposed method, however, not only is the measuring light attenuated as it passes through the relatively long capillary tube owing to absorption by the sample, but scattering processes occur in the capillary tube and often portions of the measuring light are absorbed by the wall of the capillary tube, which accordingly falsities the result of the measurement. The trend in analytical separation methods, however, such as, for example, LC, CE or SFC which have already been mentioned above, is towards smaller sample volumes and, accordingly, especially also smaller volumes of the measuring chamber or the measuring region of the detector cells. For example, in separation methods, capillary tubes having a diameter of only about 5 .mu.m are used. In biotechnological applications, it is possible in this manner to detect extremely small amounts of a sample, even in the range as small as individual cells. If one assumes that, in the case of separating columns having capillary tube diameters of about 1 mm, the volume of the measuring chamber of the detection cell should be less than 10 .mu.l in order to avoid undesired spreading and re-mixing of the separated substances, it will be appreciated that upon changing to capillary tube diameters of 10 .mu.m or less, the volume of the measuring chamber of the detector cell should be in the range of about 1 nl or less.
Since the detector cells known hitherto either are not designed for such small volumes of the measuring chamber or have too poor a detection limit owing to the short interaction paths of the measuring light with the sample to be analysed, or give false results owing to scattering processes in the capillary tube or absorption in the capillary tube wall, it is desired to provide a detector cell that, on the one hand, exhibits the necessary small volume of the measuring chamber and, on the other hand, has a sufficiently long interaction path of the measuring light with the sample. The detector cell is to be easy to use and, especially, is also to be capable of being used in miniaturised separation apparatus. Apart from having as compact a construction as possible, the detector cell is to be capable of being manufactured as simply, as reproducibly and as economically as possible. The measuring chamber of the detector cell is not to absorb the measuring light. The length of the interaction path of the measuring light with the sample --the optical path length--is to be reproducible. In addition, there is also a desire for a small detector cell that allows a continuous measuring process. All of these objects and still further objects are achieved by a detector cell according to the present invention.